Effect of water vapor pressure and temperature on the amorphous-to-crystalline HA conversion during heat treatment of HA coatings.

X-ray diffraction was used to characterize the increment of crystallinity of HA coatings after heat treatment. Coatings were heated over the temperature (T) interval of 300 degrees-460 degrees C with a partial water vapor pressure of 0.01 MPa and 0.001 MPa. Heat treatment also was done in air, as a contrast. It was found that the ratio (n) of the increment of crystallinity to the crystallinity of the as-received HA coatings was more significant for the coatings heated in atmosphere with water vapor than for those heated in air. This ratio also increased with water vapor pressure. The logarithm of the ratio increased linearly with 1/T, indicating that the ratio is exponential to T. The reason might be that recrystallization of the amorphous phase is a diffusion controlled process; the nucleation rate and growth velocity of the crystallites are in proportion to the diffusion coefficient, which is exponential to the temperature (T). Incorporation of water vapor in the atmosphere during heat treatment may decrease the activation energy for diffusion, which helps raise the diffusion coefficient of the atoms. Thus recrystallization of the amorphous phase can be accelerated.